The invention relates to solenoid systems for the actuation of engine valves. More specifically, the invention relates to methods for minimizing wiring and electronic complexity in such solenoid systems while retaining full functionality, including an ability to latch solenoids for both valve-open and valve-closed positions and an ability to control armature trajectories for quick transition with controlled impact velocity.
Solenoid systems for electromagnetic actuation of engine valves are well known in the art. These systems are required to move a valve shaft between open and closed latching positions that are relatively far apart (e.g., one centimeter), completing each transition in a short time interval (e.g., 3 milliseconds or less). The most commonly seen and successful designs rely on a single armature traveling between two independently-controlled magnetic yokes, each yoke including its own separate electrical winding powered by a separate drive circuit. In U.S. Pat. No. 6,249,418, Bergstrom describes systems and methods whereby servo-controlled actuation of each of the two yokes is controlled entirely via pairs of conducting wires, one pair per yoke. Interpretation of the relationships among current, voltage, and time for each pair of wires is used to calculate the mechanical armature position to be controlled, leading to closed-loop servo control without separate sensing hardware or wiring. Even taking full advantage of the controller taught by Bergstrom, however, two independent sets of circuitry, independently connected to the two yoke windings, are required for full control of a dual-latching actuation system.
For an electric valve actuation system developed for Sagem, in European Patent EP0992658, Thierry et. al. describe a simplified actuation system achieving solenoid action of a single armature with latching in either of two positions. A single winding creates a magnetic potential difference across space, i.e., north and south magnetic poles in separate locations partially enclosing a gap space. Each of two curving jaws of the yoke carries a magnetic polarity, one jaw at north polarity and the other at south. Each of the jaws meets one end of the moving armature in either of two axial latching positions. When the armature is far off-center near one of these latching positions, magnetic forces predominate across the smaller yoke-armature gap on the side close to latching, giving rise to a strong force toward completed closure and latching on that side. Thus, application of current to the single winding can be used to latch the armature in either of two positions. There are two significant drawbacks to the invention taught by Sagem. First, the geometric constraints of bringing magnetic flux down from a winding on the top end of the solenoid (with the valve on the bottom end, opposite the winding) result in a substantial increase in the footprint area of the solenoid, as compared to comparable conventional solenoids with separate windings. Space is required for the flux cross-section to bring flux down to the bottom latching poleface area. Further space is required to provide an adequate gap between the armature and the vertical portions of the yoke, those portions conducting flux from the winding above to the lower latching poleface surfaces. Narrowing this gap causes high leakage of flux across the armature for all axial positions in the armature travel, resulting in flux that creates no axial attraction for moving the solenoid armature along its intended travel axis, but flux that nevertheless uses flux-carrying capacity in both armature and yoke. The non-functional flux results in added winding inductance. The second drawback, related to the first in engineering tradeoffs, is that the leakage flux across the armature in its middle range of travel is quite large for any practical gap allowance between the armature and the flux-conducting yoke bridges between the upper and lower poleface areas for attraction and latching. Leakage flux uses valuable and limited flux-carrying capacity, lowering the maximum axial force achievable within yoke saturation limits.
In light of the drawbacks and limitations of the prior art, it is an object of the current invention to generate magnetic flux separately in upper and lower magnetic yokes of a dual-latching valve actuation solenoid, avoiding ferromagnetic flux bridges from top to bottom, but employing a single winding or interconnected set of windings, operated from a single pair of electrical terminals. It is an object, in one embodiment of the invention, to generate magnetomotive force for latching in both top and bottom armature positions, using a single winding that surrounds the armature above, below, and across either end, thus concentrating magnetomotive force maximally in the armature and reducing flux that leaks between yoke parts without bridging between yoke and armature. In other embodiments of the invention, it is an object to create a single effective winding including series-connected parts in both upper and lower winding window areas of the yoke, thus driving and generating flux in both yokes from a single electrical circuit. It is a related object to configure a dual-latching solenoid so that magnetic flux flowing in the wider-gapped side of the solenoid is minimized. In the context of any of the above physical and electromagnetic embodiments, it is an object to use current and voltage information from the operation of the single effective winding to determine the time that the armature crosses a central location of minimum inductance, and from information involving the value and variation of current and voltage at that crossing, to determine the flux linkage and velocity of the armature in passing that location.
A common solenoid design uses a single armature and two separate yokes, each with a separate winding and separate drive circuitry, for moving the armature back and forth and for latching the armature in a first latching position against the first yoke, or a second latching position against the second yoke. Thus, the solenoid has four electrical terminals, two for each coil, or a minimum of three terminals if the two coils share a common voltage, e.g., ground potential. Separate control of electrical excitation of the two yokes is not always necessary, however. A saving in cost and complexity is obtained if the dual-latching solenoid is configured as a two-terminal device, behaving like a single load for a single driver circuit. When control is incorporated, that driver circuit may consist of a single voltage drive with current sensing, or alternatively as a single current drive with voltage sensing. The solenoid then has one effective coil circuit, even though that one coil circuit may include series connection of two winding regions, one for each of the two yokes. This configuration would appear to entail a considerable loss of efficiency, as well as control problems. As is shown in the following Specification, however, a one coil configuration for a dual latching solenoid has unsuspected advantages.
An advantageous embodiment of the one coil solenoid is illustrated in FIGS. 1a and 1b. Two U-core yokes attract a single armature to either of two latching positions. A single winding loops through both yokes and around the ends of the armature with each turn. The armature latches efficiently in contact with either of the two yokes. The system is made unsymmetrical by designing the mechanical spring restoration system to have a neutral point some distance away from the point of magnetic force balance, so that a current flowing through the winding exerts a force to move the armature away from that neutral point. Rhythmic application of coil current at a natural resonance frequency of the armature, its payload (e.g., a valve), and the spring system, makes it possible to excite the armature to a large amplitude oscillation and latch it. Once latched, the system can be released for re-latching on the opposite side. A controller, for example having a voltage driver and current sense circuitry (as drawn) can move the armature and detect when the armature passes a reference position of minimum solenoid inductance, obtaining timing information useful for control. The controller can also determine the absolute flux linkage, that is, the flux electromagnetically linking the winding turns (henceforth commonly referred to simply as xe2x80x9cfluxxe2x80x9d), at this point of crossing over minimum inductance position. Other embodiments are shown, using separate yokes with separate windings, one for each yoke, but with the windings interconnected in series to form one effective coil circuit.